So what is the validity of my yellow fever certificate? How long does it last ? Is my old yellow fever certificate - that says it is expired - still going to work to enter various countries?
Short answer is certificate is valid for life
From World Health Organisation (WHO ) "Nothing needs to be modified in the certificate; indeed under the IHR ( International Health Regulations), any changes, deletions, erasures or additions may cause a certificate to be rendered invalid."
For the purposes of an entry requirement, in May 2014, the IHR were amended to state a lifetime validity of an International Certificate of Vaccination or Prophylaxis (ICVP) after administration of a yellow fever vaccination.
Effective July 11, 2016, the ICVP for yellow fever vaccination will be valid for life, and this validity applies to both existing & new certificates for the purposes of international travel. Revaccination or a booster dose of YF vaccine cannot be required of international travelers as a condition of entry into any country regardless of the issued ICVP date; validity begins 10 days after the date of vaccination. On new ICVPs, "life of person vaccinated" should be entered in the validity space on the certificate.
So don't lose that little yellow book!
There are circumstances where some persons may be recommended a booster of the yellow fever vaccine for their protection from the disease. Some countries are very high risk ( e.g. Angola has a big outbreak at time of writing ) or some persons may have a weakened immune system so may need multiple doses of vaccine for it to be effective - speak to your travel doctor about this.
From WHO YF Validity 2016